1. Field of the Invention
The present invention relates to a method and apparatus for bending glass sheets.
2. Discussion of the Background
Glass sheets are heated to a bending temperature (normally around 600.degree. C.-700.degree. C.) to be formed into a bent shape from a flat shape. The forming can be carried out by several measures. As one example of the measures, there is a method wherein glass sheets carried on ring molds are conveyed into a heating furnace to sag under their own weight so as to be bent, following the profile of the molds (hereinbelow, referred to as the gravity using bending method).
EP-A-621244 contains a reference to a forming apparatus with the gravity using bent method applied thereto. The forming apparatus for glass sheets controls a conveyance commencing position of molds in a heating furnace into an annealing zone when glass sheets are conveyed into the annealing zone provided subsequently to the heating furnace. Such control can adjust the degree of deformation of the glass sheets between the heating furnace and the annealing zone and provide the glass sheets with a certain bent shape.
On the other hand, the bent shapes of glass sheets for automobile windows are required to be suited to small-quantity of multi-product production in order to cope with the tendency that automobiles are manufactured in small-quantity of multi-product production. Since a heated and softened glass sheet follows the profile of its related mold, the bent shape of each glass sheet is changed by changing its related mold. The glass sheets are deformed even when the glass sheets are conveyed from the heating furnace into the annealing zone. This means that the shape of each glass sheet can be also changed by modifying the conveyance commencing position that each of the glass sheets starts moving toward the annealing zone in the heating furnace and thereby modifying the period wherein each of the glass sheet is moving.
However, in order to use the apparatus disclosed in the publication to cope with the demand for small-quantity of multi-product production, the conveyance commencing position of the molds from the heating furnace into the annealing zone is required to be modified, depending on a desired shape of the glass sheets. Modifying the conveyance commencing position as required involves complication in the driving device for the modification. In addition, since the glass sheets are successively conveyed into the heating furnace, a preceding glass sheet interferes with the moving of the succeeding glass sheet if the conveyance commencing position is modified for each of the glass sheets. In case of the apparatus and the method disclosed in the publication, it has been difficult to cope with frequent modification of the shape of the glass sheets, which are required for the small-quantity of multi-product production of the glass sheets.
The heating temperature in the heating furnace for bending the glass sheets has been experientially set based on a targeted degree of curvature of the glass sheets. Before bending glass sheets in mass-production, sample glass sheets are formed in a bent shape in the heating furnace, and the bent shape of the formed sample glass sheets is compared to a targeted degree of curvature. When the bent shape is out of an acceptable range, the heating temperature is modified by feedback.
Such method for bending glass sheets requires to check on an actual apparatus whether the bent shape of glass sheets is in the acceptable range since the heating temperature in the heating zone has been experientially set. The conventional method for bending glass sheets has a disadvantage in that considerable labor is involved until glass sheets are actually mass-produced. In particular, unlimited considerable labor is involved to cope with the small-quantity of multi-product production since checking on an actual apparatus is required for each of targeted shapes. It takes a lot of time to change the shape of the glass sheets to another one (job change).